#app {
    .box {
        display: flex;
        width:100px;
        height: 100vh;
      .left {
          float: 1px;
          background: :red;
  
      }
  
      .mid {
          flex:1;
          background-color:#ddd;
  
      }
  
      .right {
        flex:1;
        background-color: aqua;
      }
    }
  }